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Sensing current to predict and manage thermal events

As more and more systems are becoming electrified, system thermal management has become more critical. Many systems utilize temperature measurement for managing the system thermal performance. While this is adequate for many systems, for systems with rapidly changing temperature, this could be difficult. This is due to the thermal equilibrium principle which temperature measurement is based on. For systems with potentially rapid changing temperature, measuring the system current may enable a faster method for predicting and managing the thermal performance of the system.

System thermal management

Whether it is the electrification of motor vehicles, the proliferation of the Internet of Things (IoT), or Industry 4.0, more electronic systems are in the world today that are expected to be available 24/7. To ensure these systems continue to operate optimally, the system’s thermal performance needs to be monitored and managed. For many systems, a thermistor or temperature sensor integrated circuit (IC) is used to perform this monitoring. This thermal management is performed typically for one of the following use cases:

  • Long-term system reliability
  • Real-time system performance and utilization
  • System safety
  • Fault identification and prevention

As I discussed in Signal Chain Basics #86: Fundamentals of Temp Sensors, these devices work on the basic principle of thermal equilibrium, two bodies thermally connected will reach a common temperature based on the thermal mass of each. In electronic terms, this is a relatively slow process. This temperature increase is normally due to an increase of the system current flow. Systems that may be exposed to rapid increases in temperature may struggle to best manage their thermal profile if using a temperature measurement system. Therefore, measuring current may be a better alternative, especially for addressing system safety or fault identification and prevention use cases.

System safety

If left unchecked, increasing load current eventually causes key components in a system to exceed the absolute maximum junction temperature ratings. In the most severe case, the IC can actually ignite from the heat. This type of catastrophic failure is rare, but the consequences can be costly in terms of system damage, and potentially could lead to bodily injury. Monitoring the current can enable the management system to identify possible issues well before they become dangerous and allow it to take preventive actions. In the most adverse of conditions, it can simply shutdown the system with the goal of reducing current flow to below dangerous levels.

An example of prevention of bodily injury would be in the personal vaporizer. The heating of the liquid is intended to be very rapid to optimize the user experience. However, if the liquid gets too hot, the resulting vapor could cause burns to the user. By monitoring the current flow to the heating element, a current sense amplifier can allow the management system to prevent the temperature of the vapor from exceeding the recommended safe levels.

Fault identification and prevention

If a system has a fault such as a short to power or ground, serious system failures and damage are possible. Therefore, it is critical to detect faults as early as possible. Clearly waiting for the corresponding increase in temperature related to the sudden increase in current that results from a short may be too late to prevent damage. For example, many integrated gate bipolar transistors (IGBTs) used in motor control systems or DC/DC converters have integrated temperature sensing elements. However, the slowness of the reaction time of this measurement is only fast enough in high-power systems to limit the destruction to the IGBTs themselves. Using current measurement techniques to detect the rapidly rising current could enable the management system to prevent the destruction.

Detecting over-current events

Depending on the system implementation, there are multiple options for the current measurement solution. The simplest is an over-current detection device, such as the Texas Instruments INA300. This offers a very simple to implement over-current detection only scheme.

Figure 1

The INA300 offers is a very simple to implement over-current detection integrated circuit.

The INA300 offers is a very simple to implement over-current detection integrated circuit.

The voltage developed across the input shunt resistor is compared to the voltage generated across RLIMIT . When VSHUNT is greater than VLIMIT , the ALERT pin is asserted and the system management processor will take then take action to protect the system.

If the system must know the current level as well as have an over-current detection capability, a typical solution would be to use a discrete current sense amplifier and discrete comparator. This same functionality is available today in many integrated solutions such as the Texas Instruments INA301. It integrates an ultra-fast comparator with a high-accuracy current sense amplifier.

Figure 2

The INA301 integrates both a high-performance, analog-output current sense amplifier with a fast comparator.

The INA301 integrates both a high-performance, analog-output current sense amplifier with a fast comparator.

This solution gives the system management more control and flexibility on how and when it may take action. The over-current detection works just like discussed above. A typical protection scheme would not monitor the current until an over-current event occurs as indicated by the asserting of ALERT. Once this occurs, the management controller can then monitor the actual current level. It can determine if the current level is rapidly increasing indicating a potential catastrophic event is imminent. Conversely, if the current is slowly rising or fluctuating around the LIMIT, then no immediate action may be required and monitoring will continue until the over-current event either clears or does indicate a more serious problem may be occurring.

Summary

Measuring the current in a system provides a leading indicator of potential out-of-range events. It will enable the system to predict potential catastrophic events before they occur and allow for protection of critical components. Whether the concern is system performance, system reliability, fault identification of basic safety concerns, knowing as early as possible of a potential issue enables minimizing system downtime or worse!

2 comments on “Sensing current to predict and manage thermal events

  1. michaelmaloney
    October 21, 2018

    It will not be a fair experiment if the only common factor is the exact one that fluctuates. They would have to figure out an alternative setup that will enable the system to still work without having to rely on the sole factor of thermal alone. It isn't fair to rely on a formula that is not stable, thus producing results that lack credibility and truth.

  2. RituGupta
    February 26, 2019

    I think that what's really going to matter here is how the system is going to respond to increasing temperatures. Finding out that temperatures are climbing , but still useless if you can't actually carry out any actions to regulate the whole system again after you've found out.

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